Problem: 8 binders cost $12.88. Which equation would help determine the cost of 7 binders?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 7 binders. Since 7 binders cost $x$ , we have the following proportion: $\dfrac{7}{x}$ We can write the fact that 8 binders cost $12.88 as a proportion: $\dfrac{8}{\$12.88}$ The cost changes along with the number of binders purchased, and so the two proportions are equivalent. $\dfrac{7}{x} = \dfrac{8}{\$12.88}$